This invention relates to a method and system for positioning subsea work packages and, more particularly, to a method and system for directing placement of large subsea work packages in offshore applications with a direct line from a surface vessel which is subject to wave action.
Effective placement of large subsea work packages are increasingly necessary for meeting the challenges of the deepwater environment. Representative applications include carrying and placing maintenance equipment and/or replaceable modules such as control pods onto subsea completion facilities for oil and gas wells, maintaining and repairing pipelines, and retrieving previously placed subsea components. As the need for new sources of oil and gas push operations into deeper water, such operations will increasingly require exacting placement of work packages 2,000 feet or more below the ocean's surface.
The size and mass of the subsea work package renders placement by divers very difficult, and the water depth in many applications absolutely precludes the use of divers. Similarly, the size and mass of many work packages precludes direct placement with free swimming remotely operated vehicles (ROVs). Buoyancy modules might assist ROV operations, but the mass of the work packages and the size of the their required buoyancy may nevertheless preclude primary positioning operations with ROVs.
Directly lowering the subsea work package from a surface vessel on cables or other lines is well suited to accommodate the size and mass of large work packages. However, normal sea conditions subject the vessel to heave, thereby causing the vessel to fall and rise with the passing waves. Absent an effective active heave compensation system, the vessel's motion is transmitted directly through the line to the subsea work package. This uncontrolled vertical motion proves unsatisfactory for many applications and has prevented final efforts by either divers or ROVs to guide and land the subsea work packages so presented.
Attempts have been made to dynamically compensate for the heave at the line, either by driving hydraulic rams or by driving a winch as necessary to take in or pay out line to maintain the subsea work package substantially stationary despite movement of the vessel. However, such systems are expensive, complex, subject to substantial maintenance requirements and require delicate balance to operate effectively.
Therefore, there remains a substantial need for a solution to the problem of placing subsea work packages which is simple, straightforward, and otherwise suitable for real application in the offshore working environment.